A two-tube shock absorber of this kind is known from the international patent application No. WO 89/09891. A particular feature of this prior art two-tube shock absorber is its shock absorbing power that is predetermined exclusively by means of a two-stage controllable shock absorber valve which is disposed at the lower end of the central tube and through which the fluid flows in one direction only. With this arrangement, the pilot control stage of the shock absorber valve serves exclusively to control the main stage wherein the flow cross-sectional area determines the variance of absorbing power.
In the state of the art controllable shock absorber however, a disadvantage is that beyond the electromagnetic actuating power at the pilot control stage a minimum hydraulic pressure is required in the system in order to safeguard the functioning of the shock absorber in the customary range of application. Furthermore, the control times which are achievable by this arrangement and which are too long for a quick-acting shock absorber system are less advantageous. The "soft" characteristic and the quick adjustability, which are desired for driving comfort, cannot be attained for the aforementioned reasons.
It is, therefore, the object of the present invention to provide a two-tube shock absorber of the kind discussed which overcomes the aforementioned disadvantages. Moreover, a two-tube shock absorber is provided which affords the adjustment of a special desired characteristic, wherein the shock absorbing power is dependent on the piston speed, out of a field of potential characteristic curves.
According to the invention this object is achieved in that in a lower range of piston speed the shock absorbing power is exclusively determined by the variable flow cross-sectional area of the pilot control stage, while the main stage remains closed, and in that in an upper range of piston speed the shock absorbing power is determined by the variable flow cross-sectional areas both in the main stage and in the pilot control stage.
With this arrangement, it is of advantage that in the upper range of piston speed the main stage follows the position of the pilot control stage. Due to this provision, a more elevated dynamic ratio of the valve adjustment jointly with a simultaneous reduction of the vibration tendency of the shock absorber, particularly when utilized in a moving gear control process, is achieved.
A further advantage of this invention is that the main stage as well as the pilot control stage are arranged so as to be separate from the second non-return valve in a valve housing, wherein the main stage includes a main slide valve interacting with a control edge which is configured within the valve housing. The pilot control stage includes an electromagnetically actuatable control slide valve interacting with control bores which are provided in the main slide valve. A shock absorber valve having this set-up and presenting a two-stage configuration requires less actuating capacity, and affords a better handling of more elevated absorbing power levels.
The control edge is preferably positioned in proximity to the flow-off bores which are configured in the valve housing and which are used in connection with the storage tank. The flow-off bore's size and geometric shape influence the performance characteristics of the main stage. In its upper part, the main slide valve is configured open and is furnished with slide valve bores which allow the bores to be brought into connection with the flow-off bores.
According to another preferred embodiment of the invention, the main slide valve is prestressed in the closing direction of the main stage by means of an optimum first compression spring. The main slide valve is in abutment against a stop. The first compression spring assists the closing of the main stage.
Any suction or displacement problems at the main slide valve which occur during the operation of the inventive shock absorber are preferably eliminated if the abutment of the main slide valve at the stop includes axial projections.
A further advantage of the invention is that the main slide valve is provided with restricting bores which end up in a hydraulic chamber defined within the valve housing by the main slide valve. Due to this provision, both a smooth oil supply for the pilot control stage is attained and pressure drops in the event of the passage through said restricting bores is attained, which are of vital importance for regular functioning.
An advantageous coupling of the main stage to the pilot control stage is attained in another embodiment of the invention. The control slide valve is guided in the main slide valve and may be abutted against it under the prestressing action by a second compression spring.
A particularly advantageous embodiment of the invention includes a pilot control stage which is electromagnetically actuatable by a plunger coil interacting with a permanent magnet. This embodiment comprises a control slide valve captivated to the plunger coil support by means of a captivating element wherein the captivating element head may be in abutment against the main slide valve. Due to this provision, an exact control of the absorbing power in the lower range of the piston speed and an adjustability of the shock absorber valve and, as a consequence, variations of the absorbing power are rendered possible. Simultaneously, a swinging of the control slide valve over the control bores is prevented. In this embodiment, the favorable dynamic behavior of the plunger coil is of particular advantage.
According to another preferred embodiment, the actuating unit is comprised of the plunger coil and of the permanent magnet positioned in a hydraulic chamber which is in connection with the storage tank, so that an end of the captivating element, which is fixed to the plunger coil support, is subject to the action of the pressure existing within the storage tank. The aim achieved is that the plunger coil does not undergo the pressure which exists in the storage tank. Simultaneously, the effect of a downwardly directed force is applied on the captivating element.
In the event of a failure or of faulty control of the shock absorber valve to prevent an inadmissibly elevated absorbing power in the pressure stage, a non-controllable valve is provided along the lines of the invention parallel to the main stage, or the pilot control stage, which is active exclusively in the pressure stage of the shock absorber.
In this embodiment, the valve is preferably configured in the range of the second non-return valve, and comprised of a ball interacting with a valve seat and being prestressed by a spring, and, as already discussed above, becomes effective exclusively in the pressure stage.
This embodiment will be of further advantage when the valve is arranged in a connection between the second chamber and the storage tank which is formed by bores in a disc-shaped part of the second non-return valve. In the valve housing, the bore which is provided aligns with one of the flow-off bores. The aim achieved by this inventive provision is that the volumetric stream flowing through the valve is led into the already existing storage tank, so that no additional oil reservoir will be necessary.
According to a further advantage of the invention, the electromagnetic actuation of the pilot control stage takes place by means of a proportional magnet whose armature is connected to the control slide valve by means of a captivating element, wherein the head of the captivating element is abutted against the main slide valve. The advantage of this arrangement is seen in the favorable ratio between the maximum magnetic force Fmax supplied and the required overall space, or the required mass.
According to another preferred embodiment of the invention, the main stage as well as the pilot control stage are arranged within a valve housing so as to be separate from the second non-return valve. The main stage includes a valve piston interacting with a sealing seat which is configured as a disc-shaped element of the second non-return valve. The pilot control stage includes an electromagnetically actuatable control slide valve interacting with control bores which are configured in the valve piston. The use of a seat valve having such a set-up aids in overcoming soiling problems which occur in the system.
Furthermore, it will be of advantage when the disc-shaped element jointly with the valve housing defines a hydraulic annular chamber into which the valve piston projects and which is in connection with the storage tank. The above-mentioned control bores ends up in the annular chamber. These provisions provide a simple way to combine the main stage stream and the pilot control stage stream and to convey them into a common storage tank.
Other advantages of the invention are that the main stage as well as the pilot control stage are positioned within a valve housing so as to be separate from the second non-return valve. The main stage includes a piston which is sealedly guided within the valve housing, which bears a valve closing plate and which is furnished with an axial bore. The valve housing includes a sealing seat which allows the main stage to be shut off and released by the valve closing plate and which is configured as a disc-shaped element of the second non-return valve. The pilot control stage includes a cylindrical chamber configured in the valve housing. The cylindrical chamber accommodates a control slide valve elastically prestressed in the closing direction. An axially extending cylindrical wall is provided with radial bores which interact with control bores configured in the control slide valve and which afford a hydraulic link between a chamber being defined by the piston within the valve housing, on one hand, and the storage tank, on the other hand. This arrangement allows a flow through the pilot control stage at a right angle to the direction of movement of the control slide valve, even when the valve is wide open, since the position of the pilot control stage is influenced only to a slight extent.
When the pilot control stage is actuatable electromagnetically, and the control slide valve is configured with the support of a plunger coil which comprises part of the electromagnetic actuating device, a direct control of the shock absorber valve in the lower range of piston speed will be attained.
An increase of the quality of the control process is achieved in a preferred embodiment wherein a wall is provided which defines the cylindrical chamber. The wall extends in horizontal direction and includes a stepped bore wherein a section having a smaller diameter is in the chamber and wherein a section having a larger diameter accommodates an axially slidable thrust member which interacts with the front face of the control slide valve.
As an alternative, an intermediate position of the control slide valve in the condition of rest may be provided in which a partial connection is established between the chamber which is defined within the valve housing by the main slide valve, or by the valve piston, on one hand, and the storage tank, on the other hand. The intermediate position of the control slide valve is achieved by a third compression spring which counteracts the second compression spring prestressing the control slide valve.
A compact design is, finally, achieved in the aforementioned two-tube shock absorber in that the third compression spring is supported at the horizontal wall defining the cylindrical chamber.
In a further feature of the invention, an electromagnetic actuating unit for the shock absorber valve of the inventive controllable two-tube vibration absorber is disclosed which comprises a closed magnet frame which accommodates a permanent magnet as well as an electric coil interacting with the permanent magnet. In order to achieve a perfect magnetic screening effect, the range of the magnet frame carrying the magnetic flux is configured in the shape of a hollow ring in which the permanent magnet is disposed in such a way that its magnetic power flux is split into two partial fluxes directed opposite to each other. In this manner, the electromagnetic system is very effectively screened off outside and inside so that the interior space of the magnet frame is free of magnetic field of force. The interior space may thereby be used for the incorporation of components of the shock absorber valve, components of sensors and other components. The purpose achieved by the division of the power flux is that the wall thicknesses of the grounding means constituted by the magnet frame may be considerably reduced.
An advantage of the inventive actuating unit is a coil configured as a plunger coil, wherein a coil support is guided in guide slots provided in the lower part of the magnet frame and is positioned outside the magnet. The coil support is connected to a coil form bearing the winding of the electric coil by means of guide pins which extend through openings in the magnet frame. In this configuration, the plunger coil may be radially encircled by the permanent magnet or may radially encircle the permanent magnet.
In a further embodiment of the invention, ring-shaped supporting elements formed of magnetically non-conductive material fix the permanent magnet in position.
In another embodiment of the inventive actuating unit, the electric coil is unmovably positioned in the magnet frame, and the permanent magnet is fixed to the wall range of the magnet frame which radially internally defines the hollow ring and is axially movably guided in guides of magnetically non-conductive material.
In another embodiment of the actuating unit, a safe guidance of the moving coil is achieved in that the coil support of the plunger coil comprises a cylindrical range which is guided in the wall range of the magnet frame radially internally defining the hollow ring.
Another embodiment of the invention operates with a magnetic field of force comparable to the radially magnetized ring-shaped permanent magnet. In this embodiment, the wall range radially internally defining the hollow ring is configured in the shape of a polygon, wherein the front surfaces are axially magnetizable permanent magnets. The permanent magnets are cuboid-shaped, circular disc-shaped or ring segment-shaped.
An especially compact embodiment of the inventive actuating unit is distinguished in that the electric coil is formed by a single-layer winding of metal ribbon. In this embodiment, the metal ribbon may preferably be comprised of a unilaterally insulated strip of copper foil or of a superficially oxidized strip of aluminum foil.
Further details, features and advantages of the invention will be revealed by the following description of six embodiments, making reference to the accompanying drawings.